3-hydroxypropionic acid (3-HP) is a three carbon carboxylic acid identified by the U.S. Department of Energy as one of the top 12 high-potential building block chemicals that can be made by fermentation. Alternative names for 3-HP, which is an isomer of lactic (2-hydroxypropionic) acid, include ethylene lactic acid and 3-hydroxypropionate. 3-HP is an attractive renewable platform chemical, with 100% theoretical yield from glucose, multiple functional groups that allow it to participate in a variety of chemical reactions, and low toxicity. 3-HP can be used as a substrate to form several commodity chemicals, such as 1,3-propanediol, malonic acid, acrylamide, and acrylic acid. Acrylic acid is a large-volume chemical (>7 billion lbs/year) used to make acrylate esters and superabsorbent polymers, and is currently derived from catalytic oxidation of propylene. Fermentative production of 3-HP would provide a sustainable alternative to petrochemicals as the feedstock for these commercially-significant chemicals, thus reducing energy consumption, dependence on foreign oil supplies, and the production of greenhouse gases.
3-hydroxypropionate dehydrogenase (3-HPDH) is an enzyme that converts malonate semialdehyde to 3-HP (FIG. 1). Certain 3-HPDH enzymes utilize the cofactor NADP(H) (EC 1.1.1.298). However, it may be desirable with some engineered metabolic pathways for 3-HPDH to utilize the cofactor NAD(H) rather than NADP(H) (e.g., to improve redox balance). Accordingly, there is a need in the art to develop dehydrogenase variants that have increased specificity for the cofactor NAD(H) compared to NADP(H). Described herein are dehydrogenase variants that meet this need.